Studies of dopamine receptors had established that two pharmacologically distinct subtypes of dopamine receptors are expressed in the CNS, D1 and D2 receptors. Recent molecular genetic studies have now identified at least five distinct genes that codes for members of the family of G protein-coupled receptors that are activated by dopamine. The products for these genes possess pharmacologic properties similar to either D1 or D2. The genes for D1a and D1b receptors have been cloned and sequenced. These two receptors have been shown to share structural and pharmacologic properties. Studies on the in vivo regulation of dopamine receptors have thus far focused on D1 and D2 receptors because of the availability of high-affinity antagonists that can be used to differentiate between D1 and D2 and because these two dopamine receptor subtypes are expressed with the highest receptor density. Studies on the regulation of the other dopamine receptor subtypes are hampered by the lack of regents that can be used to selectively quantitate the expression of these receptor subtypes. This proposal describes experiments designed to develop receptor specific polyclonal and monoclonal antibodies and riboprobes that can be used to quantitatively measure the expression of the rat D1- like dopamine receptor subtypes D1a and D1b and the mRNA coding for these receptors. Receptor specific reagents will be used to determine relationship between dopamine receptors in the brain and in the periphery, for receptor purification studies to investigate how antagonists bind to dopamine receptors, and to define how the administration of antipsychotic and antihypertensive drugs influence the in vivo expression of D1a and D1b receptors and receptor mRNA. A solution hybridization/ribonuclease protection assay and immunological techniques will be used to quantitate the levels of receptor mRNA and the expression of receptors, respectively.